Human RNase H1 is associated with P32.

<p>(<b>A</b>) Western blot analysis of cell lysates and immunoprecipitated samples show Flag-tagged RNase H1 and H2 expression from cells stably transformed with RNase H1 (H1) or H2 (H2) or wild type (control) HEK cell lines. (<b>B</b>) Co-selection of RNase H1 binding proteins by immunoprecipitation. Extracts from cells expressing the Flag-H1, Flag-H2, or HA-H1 cell lines were immunoprecipitated with either anti-Flag or anti-HA antibody. Co-precipitated proteins were resolved by SDS-PAGE, and visualized by silver staining. Protein bands that were different from the co-precipitated proteins from control cells were subjected to mass spectrometry. The protein bands corresponding to the tagged RNase H1, H2 and the co-precipitated P32 proteins are indicated. The size marker was SeeBlue Plus2 Pre-Stained Standard (Invitrogen). (<b>C</b>) 2D gel electrophoresis of proteins co-precipitated with Flag-H1 or Flag-H2. About 5 mg cell lysates were prepared for immunoprecipitation with anti-flag beads from cell lines which stably express Flag-H1 or Flag-H2. The immunoprecipitates were washed four times with RIPA buffer and directly sent to Applied Biomics Inc. (San Francisco, CA) for 2D gel electrophoresis coupled with MS analysis. In brief, the co-precipitated proteins from Flag-H1 or Flag-H2 cells were labeled by fluorescent DIGE CyDyers, respectively, followed by 2D gel electrophoresis. The protein image was scanned with a fluorescence detector. The figure illustrates the proteins differentially associated with RNase H1 (green) or H2 (red). The P32 protein was confirmed with mass spectrum from the extracted gel sample. Circled spots were identified as RNase H1, H2 or P32 by mass spectrometric analysis. (<b>D</b>) Both endogenous and expressed RNase H1 are co-precipitated with the expressed P32. Left panel: western blots with P32, RNase H1, or H2 antibodies for proteins co-precipitated using anti-HA antibody from extracts of control HeLa cells or cells transfected with HA-P32 expression plasmid. Right panel: western blots for proteins co-selected using anti-HA antibody from extracts of Flag-H1, Flag-H2 stable cell lines and control cells, all of which were transfected with HA-P32 expression plasmid. (<b>E</b>) Confirmation of the specific interaction between RNase H1 and P32. RNase H cleavage activity indicates that the P32 co-immunoprecipitated material contains only RNase H1 enzyme activity. Upper panel: Cleavage patterns of human RNase H1 and H2 from IP-coupled enzyme activity assays. Immunoprecipitations were performed with either anti-flag, anti-RNase H1 or anti-H2 antibodies from extracts of Flag-H1, Flag-H2 expressing cells or control cells. The co-precipitated samples were incubated for the indicated times with a ³²P-labeled RNA/DNA-methoxyethyl (MOE) gapmer duplex and the cleavage products were separated using denaturing gel electrophoresis. The preferred cleavage sites of RNase H1 and H2 are indicated with * or #, respectively. The positions of the preferred cleavage sites in the heteroduplex are shown in the middle panel with the sequences of the RNA substrate (upper strand) and the oligonucleotide (lower strand). The bold nucleotides in the oligonucleotide strand indicate the position of the MOE substitutions. Lower panel: only the RNase H1 enzyme activity was detected in the co-precipitated material from lysates containing tagged P32. Immunoprecipitations were performed with anti-HA antibody from extracts of Flag-H1 or Flag-H2 stable cell lines or control HEK cells, which were all transfected or not transfected with HA-P32 expression plasmid. The precipitated samples were analyzed for cleavage patterns as described above. The position of the cleavage bands relative to the sequence of the cleavage products is shown on the left. A partial alkaline digestion of the same labeled RNA was used as a sequence ladder. The cleavage pattern of purified human RNase H1 is shown at the far right of the lower panel.